Skeletal muscle represents the major repository of body protein. In addition to its functional role, skeletal muscle provides amino acids for processing by visceral organs during times of stress and nutrient deprivation. Caloric or protein deficiency may result in less than optimal skeletal muscle energy metabolism thereby affecting muscle function before changes in structure or composition can be detected. We hypothesize that variation in muscle high energy metabolism at rest, during mild to moderate exercise and during recovery from exercise may be used to characterize the various nutritional and metabolic states associated with nutritional depletion and injury. These changes may be used to understand the effects of nutritional support in surgical patients and to determine the optimal nutrient requirements for skeletal muscle. Phosphorus nuclear magnetic resonance spectroscopy (31P NMR) will be used to study the effects of malnutrition on high energy phosphate metabolism in skeletal muscle. The relative quantities of ATP, phosphocreatine, and inorganic phosphate will be measured and the rate of reconstitution of the high energy pool will be monitored before, during and after standard work loads. NMR techniques will also be used to measure creatine kinase activity in vivo, pH changes, free ADP concentrations, and the phosphorylation potential. The effects of caloric protein deprivation, refeeding, operative stress and an anabolic hormone will be studied.